The present invention generally relates to power converter controllers and more specifically to a current mode controller utilizing a time compensated current ramp which prevents imbalance of the inverter current of the power converter.
Power converters are known in the art and typically serve to accept energy from an unregulated energy source, such as a voltage source, and generate a regulated voltage which is applied to a load circuit. A switching device is typically used to control the transfer of energy to the load circuit in order to provide the regulated voltage. The switching device typically has two modes of operation, fully on and fully off. The switching device is periodically turned on for a time interval to permit energy transfer for the purposes of maintaining the output voltage at a predetermined level.
Typically, the switching type regulating devices employed in power converters utilize semiconductor devices such as power transistors as the switching device. These devices are turned fully on, or saturate, or fully off during operation. When fully on, the semiconductor devices are conducting and little or no power is dissipated. When nonconducting or fully off, the semiconductor devices do not dissipate power. However, power is dissipated in the semiconductor devices during the time interval of switching from nonconducting to conducting and vice versa. It is during the switching time interval that a substantial amount of power may be dissipated in such a semiconductor device.
It is desirous to provide power converters of the switching type which are cost effective, occupy small amounts of space and convert large amounts of energy efficiently. In order to achieve this goal, high frequency/high power switching power converters with a series resonant circuit and sinusoidal operating current are used. In such series resonant power converters, the on switching device is only turned on when minimal or zero current is flowing through the resonant circuit. The off switching device is turned on based on load requirements. Typically, the current of the series resonant circuit is used to control the operation of the on and off switching devices.
However, a problem arises when using the current mode control because of imbalance between the conduction period of alternate half cycles of the series resonant power converter. Specifically, the conduction period of each half cycle is terminated when the current ramp (integrated sample of the primary current) reaches the level of the error signal. The error signal represents the error between the output voltage and a fixed reference. Any difference in the initial condition of the components of the series resonant circuit, at the beginning or alternate conduction periods causes a change in the primary current. The result is a difference in conduction times for the primary current to reach the same average value. In other words, there will be multiple inverter operating points for the same ramp amplitude. Additionally, the prior art current mode control of series resonant power converters suffers from control problems at light load conditions. This occurs due to the fact that the slope of the current ramp is very shallow at the beginning of the conduction cycle for light loads.
The present invention addresses the above-mentioned deficiencies in the prior art by providing a current mode, series resonant power converter which can be used over a very wide ranges of loads, including instances where no external load is provided. Additionally, the present invention provides a control which prevents the problems of conduction time imbalance associated with the prior art current mode controllers.
A current mode controller for a series resonant power converter having a primary current, a conduction time and a pulse width modulator controller is disclosed. The current mode controller having a current ramp generator in electrical communication with the primary current and being operative to generate a current ramp having a voltage proportional to the primary current. A voltage ramp generator is in electrical communication with the power converter and operative to generate a voltage ramp. The current mode controller further includes a multiplier in electrical communication with the current ramp generator and the voltage ramp generator. The multiplier being operative to generate a time-dependent current ramp having an amplitude proportional to the conduction period of the power converter. A limited voltage ramp generator operative to generate a limited voltage ramp is in electrical communication with the voltage ramp generator. In this respect, the current mode controller further includes an adder in electrical communication with the limited voltage ramp generator and the multiplier and operative to generate a controlling ramp proportional to the limited voltage ramp and the time-dependent current ramp. As such, the adder is in electrical communication with the pulse width modulator controller such that the controlling ramp controls the conduction period of the power converter.
In accordance with the present invention, the series resonant power converter has a drive signal and the current ramp generator has a rectifier operative to generate a rectified primary current and a first capacitor in electrical communication with the rectifier. In this respect, the first capacitor is operative to generate the current ramp from the rectified primary current.
In the preferred embodiment of the present invention, the voltage ramp generator has a second switch in electrical communication with the drive signal of the series resonant power converter. Additionally, the voltage ramp generator includes a second capacitor in electrical communication with the second switch. Accordingly, the second switch and the second capacitor are operative to generate the voltage ramp in proportion to the drive signal. The limited voltage ramp generator of the present invention comprises an amplifier in electrical communication with the voltage ramp in order to generate the limited voltage ramp. In the preferred embodiment, the controlling ramp is configured to be compared to an error signal of the pulse width modulator.
In accordance with the preferred embodiment of the present invention, there is provided a method of controlling a series resonant power converter. The method comprises generating a current ramp having a voltage proportional to the primary current and generating a voltage ramp. Next, a time-dependent current ramp is generated from the current ramp and the voltage ramp. The time-dependent current ramp has an amplitude proportional to the conduction period of the power converter. A limited voltage ramp is then generated from the voltage ramp previously generated. Finally, a controlling ramp is generated from the limited voltage ramp and the time-dependent current ramp. The controlling ramp is proportional to the limited voltage ramp and the time-dependent current ramp such that the controlling ramp controls the pulse width modulator controller.
In the preferred embodiment, the current ramp is generated by a current ramp generator and the voltage ramp is generated by a voltage ramp generator. The time-dependent current ramp is generated by multiplying the voltage ramp with the current ramp. The controlling ramp is generated by adding the limited voltage ramp with the time-dependent current ramp.